1. Field of the Invention
This invention relates generally to surge control for compressors and, more particularly, to reducing the incidents of surge and improving turndown in centrifugal compressors or the like used in compressed gas systems.
2. Description of the Prior Art
The use of centrifugal compressors and the like in gas compression systems is well known in a variety of areas. For example, centrifugal compressors have been used to supply compressed air to one or more reservoirs which, in turn, supply the air to a plant, factory or other facility which requires a steady supply of pressurized air for tools, equipment, and the like. Such systems are normally designed to maintain a particular volume flow of air at a particular pressure. This is often referred to as the design point for the system.
Such a system will operate without the need for any particular control arrangement as long as the demand of the user of the compressed air remains at or near the design flow. A problem develops when the demand of the plant fluctuates, particularly when the demand for the pressurized air drops below the design flow level. As the flow drops, the discharge pressure of the compressor will tend to rise and the system may quickly reach the surge level for the compressor. See, for example, U.S. Pat. No. 3,901,620 which discusses the problems and characteristics of compressor surge.
A variety of arrangements have been suggested for controlling compressor surge as shown, for example, by U.S. Pat. Nos. 3,276,674, 3,424,370, 3,737,252, 4,046,490, 4,142,838, and 4,164,035. In a typical arrangement, the air flowing into the compressor is controlled by an inlet or throttle valve; excess air discharged by the compressor, as the discharge pressure increases beyond a maximum level, can be blown off by an unload valve or the like. As the system demand for the compressed air decreases, which is reflected in a decrease in current in a motor controlling the compressor, the inlet valve is gradually closed (i.e., throttled) to keep the system operating near the design pressure. However, use of an inlet valve alone has certain limitations since the system will eventually reach the surge line at lower flows and cause the compressor to go into an undesirable surge. Therefore, another control mechanism is needed whenever the flow approaches a particular minimum flow level, which is spaced a safe distance away from the surge line.
In accordance with another known system, the inlet valve is gradually closed until it reaches the minimum safe level for flow; the inlet valve is then not closed any further but remains frozen at the last position. Further reductions in air flow, as detected by a decrease in compressor motor amps, will cause the compressor discharge pressure to increase. When the discharge pressure reaches a particular maximum level above the design pressure, the unload valve, which was fully closed before that point, is fully opened and the inlet valve is fully closed. Thereafter, the control system monitors the system pressure in the reservoir to see if it drops below a particular minimum level. If so, the inlet valve is totally opened and the unload valve is totally closed in order to bring the pressure in the reservoir back up to a desired minimum level. The practice of modulating the unload valve to help control surge is also recognized in the art.
However, these systems have a number of problems. In particular, they use a high rise in compressor discharge pressure to determine whether to use a control other than throttling of the inlet valve. A risk is present in such a system that the compressor will accidentally go into surge. In addition, the minimum flow rate before using the unload valve must be spaced a large distance away from the surge line at the design pressure level to insure that surge is not reached. In addition, the characteristic curves for the operating system must be relatively steep in order to insure that surge is not quickly reached with a slight drop in flow rate. Thus, the design criteria for the compressor system becomes very crucial and the designer is given little design leeway. In addition, since the minimum flow rate must be a large distance from the crossing of the surge line with the design pressure line, the use of the inlet valve to control surge, which is more energy efficient than the use of the unload valve, is more restricted than is really needed.
Therefore, it is an object of the present invention to obtain a greater turndown in the control system, namely, more use of the inlet valve to control the system and prevent surge. It is also an object of the present invention to minimize the use of the unload valve for controlling the system and preventing surge. It is a further object of the present invention to control surge in the system but at the same time to reduce the inefficiencies of unloading large amounts of air from a running compressor and to recognize certain extreme fluctuations in demands on the system flow to minimize needless and energy inefficient operation of the compressor.